1. Field of the Invention
The present invention relates to a blue phosphor and a display panel using the same.
2. Description of the Related Art
An image display apparatus using the light emission of a phosphor is a self-emission type of light, superior in color reproducibility, high in luminance and excellent in moving image display characteristic. Such an image display apparatus has been practically widely applied as a cathode ray tube (hereinafter referred to as CRT) in the industry. In recent years there has been demand for the image display apparatus to meet the requirements for further increase of performance, size, picture quality and saving of energy and space according as image information is diversified and density is increased. Under such circumstances, there are growing expectations for the image display apparatus as a flat panel display (hereinafter referred to as FPD) which is a self-emission type of light, such as a plasma display panel (hereinafter referred to as PDP) and a field emission display panel (hereinafter referred to as FED).
A thin film transistor driven liquid crystal display device (hereinafter referred to as TFT) has already been practically applied as a FPD and has replaced a part of the market which the CRT has dominated heretofore because of its excellent characteristics such as wide color reproducibility, high luminance and long lifetime. The TFT which is not a self-emission type of light may produce an unavoidable problem with the performances of image formation such as a narrow angle of field, inferior moving image visibility and fading of a black color caused by having to use a backlight light source. Although the TFT features a low consumption power as far as it is small in size, which can be distinguished from other FPDs, the FPD has required a large consumption power for a backlight light source according as a screen size increases and has now had a problem with a high consumption power.
As described above, the FPD has problems difficult to solve in a full-scale diffusion. The FED is one of systems drawing attention as the FPD to solve the above problems.
The structure of the FED is described in brief below. Devices used as electron emitting sources are arranged in a matrix correspondingly to each pixel over a rear plate. Wirings for driving a multiplicity of the devices are arranged in a matrix. The electron sources now proposed as many embodiments include a cathode chip having three-dimensional structure like Spindt type, flat shaped type, and carbon nanotube type. The application of a voltage to these electron sources in vacuum through the wirings according to image information causes the electron sources to emit electron beams according to image information.
A face plate includes a layer formed by phosphors which emit light by an excitation source of accelerated electron beams. A high voltage is applied across the rear and the face plate to accelerate electron beams emitted from the electron source to provide the phosphors with a required excitation energy, thereby enabling forming images according to image information.
The face plate needs to efficiently remove electric charges accumulated on the phosphor layer of a substantially insulating material to efficiently reflect light emitted from the phosphor. For this reason, a metallic film called a metal back with a small atomic number such as aluminum is generally provided over the phosphor layer. Incidentally, the FED which is used in a low accelerated voltage area creates a problem with energy loss caused by the metallic film, so that an electroconductive transparent film such as indium tin oxide can be formed on the face plate.
The FED needs to be a vessel with a high vacuum of at least about 10−4 Pa because it is driven by the foregoing mechanism. For this reason, in general, a frame with an adequate thickness is interposed between the face plate and the rear plate, a plurality of components called a spacer is arranged between the plates to maintain the shape of the vessel against the atmospheric pressure and the components are bonded. Air is evacuated from the vessel to produce the vacuum vessel. The spacer is typically arranged between adjacent pixels of phosphors, or over a non-light emission black area (or a black matrix) provided to suppress the reflection of external light. In general, it is necessary to arrange a sufficient number of the spacers to support the vessel against the atmospheric pressure.
In the FED which features flat type, the space between the cathode substrate of the electron source and the anode of the front plate is typically limited to several mm, so that an accelerating voltage of as high as 25 kV cannot be used unlike CRT because of limitations of voltage endurance. In even a high-voltage type FED, an accelerating voltage is limited to 15 kV or lower. For this reason, the penetration depth of an exciting electron to the phosphor layer is inevitably shorter than that in the CRT. It is typically indispensable to use a high current density or a line sequential driving to realize a practically usable luminance equivalent to that of the CRT.
This requires a phosphor to secure luminance linearity at a high current area as well as high light emission efficiency and provide stability in luminance for electric charges applied. Furthermore, the phosphor is required to exhibit light emission high in color purity to realize a high level display device. Only a group of zinc sulfide phosphor called EIA P22 which has long been used in the CRT is known as electron beam excitation phosphor which has been practically used.
The zinc sulfide phosphor is not always sufficient in stability for electric charges applied and the time degradation in luminance is further conspicuous in the FED which is operated at a higher current area than the CRT. In addition, various problems are caused in that sulfur dissociated by thermal energy produced by applied electric charges is dispersed into the vacuum vessel to decrease degree of vacuum and exert an adverse influence on the electron source.
The above problem is particularly conspicuous in a blue phosphor ZnS:Ag which requires the highest current to obtain sufficient visual light-emission efficiency.
To tackle these problems, Japanese Patent Application Laid-Open No. 2002-265942 discloses a method of producing zinc sulfide phosphor with a little crystal defect and Japanese Patent Application Laid-Open No. 2004-307869 also discloses a process for correcting the crystal defect and surface strained layer of zinc sulfide phosphor. These proposals have provided improvements to some extent.
However, the zinc sulfide phosphor has an unavoidable problem in that a luminance linearity deteriorates at a high current area in addition to the problem with instability for applied electric charges. This is because light emission mechanism in the zinc sulfide phosphor is a second-order reaction referred to as “donor-acceptor pair light emission type” and the concentration of donor and acceptor cannot be sufficiently increased because of the problem with concentration quenching. This drawback in that attenuation time is slow is disadvantageous to high-definition vertical line driving adapted to HDTV, so that it needs to be solved.
As stated above, there have been intense demands for the development of an electron beam excited blue phosphor to be possibly substituted for the zinc sulfide phosphor and, in particular, the invention of a phosphor with non sulfide as host and 4f5d type allowed-transition luminescent center faster in attenuation time. Phosphors except the zinc sulfide phosphor have been researched so far. For example, phosphor Y2SiO5:Ce3+ has a 4f5d type allowed-transition luminescent center in its oxide host used as special electron tube such as beam indexing tube for electron beam excitation. However, the phosphor has a drawback in that the half width of spectrum thereof is wider, so that it cannot emit sufficiently pure blue.
On the other hand, the PDP is a prospective display as the next generation FPD as well as the FED. The blue phosphor practically used in the PDP at present is only aluminate phosphor of alkaline earth metal, specifically, phosphor BaMgAl16O27:Eu (BAM). The phosphor emits blue light high in luminance and excellent in color purity by vacuum ultraviolet-ray excitation, but has a serious problem with deterioration during PDP panel production process and inferiority in driving durability. For this reason, there has been a demand for the invention of a phosphor substituted for the BAM. Japanese Patent Application Laid-Open No. 2004-175786 proposes improvement on decrease in crystal defect of the BAM phosphor. Japanese Patent Application Laid-Open No. 2004-172091 proposes improvement on change of electrification characteristic of the BAM phosphor surface. In addition, for example, improvements by adding additive to the BAM phosphor have been proposed. However, the above improvements have not obtained sufficient effect. Other phosphors need to be invented.
A phosphor represented by a general formula (1):M1xM22−xSi2O6Raz  (1)(where, M1 and M2 are alkaline earth metal, x is smaller than 2, Ra is Ce or Eu and 0.005≦z≦0.05), in particular, a phosphor CaMgSi2O6 emits light with a narrow half width having 449 nm at its central peak, emits blue light excellent in color purity and is superior in process resistance and driving durability. For this reason, the phosphor CaMgSi2O6:Eu has availability as a new blue phosphor for the PDP panel capable of replacing the BAM phosphor. However, on the contrary, the phosphor has a problem in that a visual luminance cannot be sufficiently increased because of its high color purity, which urges further improvement in luminance.
Japanese Patent Application Laid-Open No. 2004-231930 attempts improvements in luminance by optimizing the grain size and halogen component having a fusing agent effect. Japanese Patent Application Laid-Open No. 2004-352936 also attempts improvements in luminance by improving baking process and Japanese Patent Application Laid-Open No. 2004-176010 attempts improvements in luminance by optimizing stoichiometric composition. Furthermore, an attempt has been made to improve the absorption in a VUV area by adding a sensitizer. None of them has obtained sufficient effect.
Thus, in the FPD such as FED and PDP, a blue phosphor needs to be invented, excellent in color purity, high in luminance and superior in durability.